Introduction

The Río Grande Rift began developing in the late Oligocene about 25 million years ago as the North American continent started to tear apart. The pace of rifting was relatively slow until about 15 million years ago, but then accelerated with rapid fault movement and volcanism in the late Miocene. The SLV dropped down along deep faults, and volcanic eruptions and magma intrusions accompanied subsidence of the valley. The resulting basin is filled with 1000s of meters of volcanic strata and sediments eroded from adjacent mountains.

A final tectonic pulse of fault movement, volcanic eruptions, and magma intrusions took place during the Pliocene Epoch, roughly 5 to 3 million years ago, when the Taos Plateau and large volcanoes were built. These events resulted in isolated basins that contained lakes, much like large lakes in the modern East African Rift system. Lake Alamosa covered much of the central SLV up to 150 feet (45 m) deep north of the San Luis Hills. The existence of this lake was first noted by Fowler in 1811-12 and later elaborated by Endlich (1877) and Siebenthal (1910), as summaried by Ruleman et al. (2016).

At its maximum, Lake Alamosa reached an elevation of 7660 feet (2330 m) and covered nearly 4000 km² (~1500 square miles). The smaller Sunshine Lake was located between Ute and Guadalupe mountains in northernmost New Mexico. By the middle Pleistocene, Lake Alamosa had largely filled with sediment. Influx of glacial meltwater from surrounding mountains led to overflowing, and the Río Grande gorge was eroded across the San Luis Hills approximately 360,000 years ago. The outlet gorge across the San Luis Hills was cut down rapidly, and water flowed out across the Costilla Plain south of the hills. Farther south in the Ute Mountain vicinity, the Río Grande began to eroded through surficial gravel deposits. By around 250,000 years ago, the river had cut down to the level of basalt, and incision of the modern Río Grande canyon began (Ruleman et al. 2016).

Also present at this site is a preserved fragment of original wall from a rectory built in 1879 to serve Jesuit Priests of the Society of Jesus; later it was converted to a convent for the Sisters of Loretto at the Foot of the Cross. A large circular feature was under construction (May 2012) between the church, rectory wall, and cemetery. Dirt work and drainage were completed along with concrete sidewalks. The Rosary Walk was completed in October 2012 and blessed by Pope John Paul II. We visited again in May of 2017 to rephotograph the site, when Toshiro Nagasako assisted with kite aerial photography.

The great height of the Sierra Blanca Massif is supported by uplifted granite approximately 1.7-1.8 billion years old, and the massif is flanked by huge alluvial fans that have coalesced along the southern and western sides (Chronic and Williams 2002). The lower portions of these fans merge into the desert floor of the SLV. The middle and upper portions rise with increasing slope toward the mountain front. These are among the most impressive alluvial fans anywhere in the world and have been the subject of considerable recent research (e.g. Johnstone et al. 2018).

Alamosa NWR hosts thousands of shorebirds and waterfowl, of which some migrate through on a seasonal basis and some nest in the refuge. Many species of ducks, geese, and cranes are especially well known—sandhill crane, Canada goose, northern pintail, American avocet, and white-faced ibis. Raptors include the bald eagle and northern harrier as well as various owls and hawks.

When Lake Alamosa stood at its highest level, Saddleback Mountain was a small island. The lake reached far westward and northward. Storm-driven waves eroded the northern side of the island, and bouldery sediment was transported and deposited as spits of beach gravel on the southern side of the island. These spits are well preserved as terraces or benches at two levels: 7660 feet (upper) and 7640 feet (lower). In fact, this site is a key location for understanding the extent and age of Lake Alamosa (Machette et al. 2007, p. 63).

Lee Trail is the last access point on the Río Grande canyon in the San Luis Valley; the rough trail descends 215 feet (70 m) from the parking area on the plateau to the river bank below. The canyon walls here are formed in lava flows of the Servilleta Basalt, as near Ute Mountain. Eruption and growth of volcanoes near Questa, beginning about 6 million years ago and continuing to ~1 million years ago, blocked local drainage and led to a lake in Sunshine Valley between Ute and Guadalupe mountains. The lake dried up by 640,000 years ago (Machette et al. 2011). Toshiro Nagasako assisted with kite aerial photography at this site.

References